Tuftsin, a naturally occurring hormone-like tetrapeptide of the sequence Thr-Lys-Pro-Arg, derives from the leukophilic immunoglobulin G. This factor binds to tuftsin receptors on granulocytes and macrophages, and stimulates their phagocytic and bactericidal activities. Postsplenectomy infections present serious problems in the management of patients who undergo a splenectomy, particularly pediatric patients under four years of age. They often experience life-threatening septicemia. It has been clearly documented that splenectomized animals and humans possess drastically reduced levels of tuftsin in their sera. In view of this, the objective of this research project is to examine if the administration of tuftsin to splenectomized mice can restore the ability in the hosts to cope with infections. Sham-operated mice, splenectomized mice, splenectomized-spleen implanted mice, and tuftsin-treated mice following a splenectomy will be compared for survivals after a pneumococcus challenge. Different does of tuftsin administration will be attempted. In order to further delineate the role of tuftsin in splenectomized mice and humans at the cellular and molecular levels, in vitro experiments will be carried out using the specimens obtained from the splenectomized hosts. Splenectomized mice, which are treated with tuftsin, will be examined by the pneumococcus blood clearance and bactericidal activity assays. In vitro experiments using blood specimens obtained from patients include examinations of the response of granulocytes to tuftsin, activity of leukophilic immunoglobulin G and blood levels of tuftsin. Tuftsin will be chemically synthesized and purified in this laboratory.